Caloric vestibular stimulation (CVS) has been widely and safely utilized for more than a century for diagnostic purposes, particularly in the emergency room to detect brain function after injury and in ear, nose and throat practice to assess balance.
CVS activates the sensory organs of the vestibular system located within the inner ear. The core elements consist of the semi-circular canals, which sense rotational motion, and the otoliths, which sense linear acceleration. Motion within the semi-circular canals is detected through motion of internal fluid (endolymph), which in turn activates hair cells that generate electrical signals, which are then transmitted via the 8th cranial nerve to the brainstem and widely throughout the cerebellum and cortical regions. In traditional CVS, irrigation of the external auditory canal (the ear canal) with warm or cold water/air changes the density of the endolymph in the semi-circular canal of the inner ear, which in turn activates the pathways noted above. Nystagmus, or the vestibulo-ocular reflex, is an easily observed result of CVS, wherein the eyes move spontaneously, even if the patient is unconscious.
There have been intriguing (but largely anecdotal) reports of using CVS as a therapeutic measure. Survey articles document a variety of outcomes and discuss some of the mechanisms involved (Miller et al., ACTA NEUROPSYCHIATRIA 19:183-203 (2007); Been, et al., BRAIN RES. REV. 56:346-36 (2007)). Squirting or blowing warm/cold water/air into a patient's ear, however, is crude, does not provide closely controlled thermal activity and is not consistent with medical dosing. Nevertheless, CVS is known to activate specific brainstem, cerebellar and cortical sites, which have therapeutic potential, as demonstrated through functional imaging (Bottini et al., EXP. BRAIN. RES. 99:164-169 (1994); Bense et al., ANN. N.Y. ACAD. SCI. 1004:440-445 (2003); Dieterich et al., BRAIN 131:2538-2552 (2008); Baier et al., HUM. BRAIN MAPPING (Sep. 2, 2009) [e-published ahead of print]; Naito et al., BRAIN 126 (2003)).
In addition, vestibular stimulation is also known to release important neurotransmitters (e.g., serotonin, acetylcholine, histamine, endorphins, vasopressin and dopamine) (Fu-rong et al., CHIN. MED. J. 120(2):120-124 (2007); Horii et al., J. NEUROPHYSIOL. 72:605-611 (1994); Tabet, AGE AND AGING 35:336-338 (2006); Horii et al., J. NEUROPHYSIOL. 70:1822-1826 (1993); Horii et al., BRAIN RES. 914:179-184 (2001)).
In contrast to both pharmaceutical treatment and neurostimulation devices which employ electrical signals, CVS appears to have an advantage: although nystagmus habituates with repetition of CVS (Naito et al., BRAIN 126 (2003), the vestibular neurological response appears not to be patient to such habituation or accommodation. (Emani-Nouri, ACTA OTOLARYNGOLOGICA 76:183-189 (1973)). In addition, CVS does not have the same potential for side effects like a drug. Yet, CVS has not attained wide-spread use for therapeutic purposes. Hence, there remains a need for new ways to carry out CVS for therapeutic purposes.